LOL.....I don't mind my advice being questioned...I am used to it on AB.
Now you are making me be more scientific ( which I hate)
A virus acts on the lining of the nose and it swells up...blocks the nose. A decongestant acts on the nose, shrinking the lining (decongests)and the airway is clear.
A virus acts on the lining of the nose causing the lining to swell AND the lining of the nose produces SNOT which cant get out because the lining of the nose is preventing it doing so. A decongestant will shrink the nasal lining and the SNOT will be able to escape, clearing the nasal passage.
Now......occasionally polyps (benign growths) or a deviated cartilage may cause the blockage and no amount of decongestive agents will improve this. One then must rely on the skill of the surgeon.